The mobility of the hands can be impaired, particularly after accidents with tendon injuries or as a result of strokes. Exoskeletons, which are fitted over the hand like a second skeleton and can move the wrist and fingers in a targeted manner, are increasingly being used as an aid to therapy. However, only a product that is completely tailored to the patient's hand and meets important requirements such as low weight and compact dimensions provides a good basis for successful therapy. After all, a support and movement structure that hinders rather than supports its wearer often collects dust on the shelf unused.

The basis: CAD modeling and innovative 3D printing

The Fraunhofer Institute for Machine Tools and Forming Technology IWU in Chemnitz is now relying on innovative shape memory alloys, stepper motors and 3D printing technologies to provide precisely fitting and gently supportive exoskeletons. The idea: the exoskeleton as such must first be perfectly adapted to the anatomy of the hand. Human hands differ significantly not only in size, but also in their proportions. There are even differences between the left and right hand of the same person. The production of the exoskeleton via 3D printing using the selective laser sintering (SLS) process should offer almost unlimited geometric freedom. In this process, three-dimensional shapes are built up layer by layer from plastic powder. According to Fraunhofer IWU, the manufacturing process is particularly efficient if it is based on a parametric CAD model. Ideally, the patient's hand is available as a digital 3D scan so that the relationships (parameters) between the individual elements of the hand are mapped along with the geometric shape. This should not only produce a particularly precise "negative" of the hand during printing, but also make it easier to make any necessary adjustments for subsequent new fabrications using a CAD model as a basis.

Compact and precise drive

But it's not just the anatomy of each hand that is unique. Hand strength also differs significantly. For most patients, it makes sense to individualize the force and stroke of an exoskeleton for the hand. At the same time, a high dead weight of the aid would be very stressful. In addition to the fit of the shell, the drive is therefore particularly important. Alina Carabello, a doctoral student at the Fraunhofer IWU and research assistant at Chemnitz University of Technology, relies on a combination of a bidirectional stepper motor and actuator wires designed as shape memory alloys. This concept is based on just two artificial tendons, which the stepper motor precisely controls for a flexion or extension movement of the hand and fingers. Various positions can be maintained and readjusted as required. For example, readjustment is required for a plastic mineral water bottle, which gives way as soon as the cap is opened and carbon dioxide can escape.

Smooth response behavior guarantees gentle movement steps

And this is how the drive system works: The stepping drive is based on the elements gear wheel, lever, pawl and SMA wire. When the wire is heated, it contracts and deflects the lever by a defined angle. This causes the cogwheel to be "entrained", i.e. moved in the desired direction; the cable of the extensor or flexor tendon is wound onto or unwound from a cable spool. This tendon movement flexes or stretches the finger. In Carabello's stepper drive, shape memory alloys (SMA) act as actuators: Their ability to change shape controls the movement of the cogwheel. If an SMA wire is heated by an electric current from a microcontroller, it shortens; when it cools down again, it stretches due to a counter-tension, for example by a spring or another SMA wire. This mechanism controls the artificial limbs of the exoskeleton. Overall, a very precise and repeatable movement can be achieved; the smooth response of the SMA wires guarantees gentle movement steps.

Individualized exoskeletons for therapeutic purposes

The Fraunhofer IWU sees a need for further individualization in many medical technology aids in order to better reflect the needs of users and provide more targeted support for therapeutic staff. Many aids are based on standard specifications and are rarely adapted to the age, gender, height, weight or actual limitations of patients, or can be adapted at all. With its exoskeleton for the hand, the Fraunhofer IWU is primarily targeting tendon injuries following accidents. After strokes or with congenital paralysis, for example, it is also important to be able to move the affected hand during the recovery process - exercises in a closer sequence than therapists' time budgets allow would often be useful. The exoskeleton developed at the Fraunhofer IWU is intended to provide relief here, as the continuous presence of a caregiver is not absolutely necessary.

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